Audio matrix encoding and decoding is well known in the prior art. For example, in so-called “4-2-4” audio matrix encoding and decoding, four source signals, typically associated with four cardinal directions (such as, for example, left, center, right and surround or left front, right front, left back and right back) are amplitude-phase matrix encoded into two signals. The two signals are transmitted or stored and then decoded by an amplitude-phase matrix decoder in order to recover approximations of the original four source signals. The decoded signals are approximations because matrix decoders suffer the well-known disadvantage of crosstalk among the decoded audio signals. Ideally, the decoded signals should be identical to the source signals, with infinite separation among the signals. However, the inherent crosstalk in matrix decoders results in only 3 dB separation between signals associated with adjacent directions. An audio matrix in which the matrix characteristics do not vary is known in the art as a “passive” matrix.
In order to overcome the problem of crosstalk in matrix decoders, it is known in the prior art to adaptively vary the decoding matrix characteristics in order to improve separation among the decoded signals and more closely approximate the source signals. One well known example of such an active matrix decoder is the Dolby Pro Logic decoder, described in U.S. Pat. No. 4,799,260, which patent is incorporated by reference herein in its entirety. The '260 patent cites a number of patents that are prior art to it, many of them describing various other types of adaptive matrix decoders. Other prior art patents include patents by the present inventor, including U.S. Pat. Nos. 5,625,696; 5,644,640; 5,504,819; 5,428,687; and 5,172,415. Each of these patents is also incorporated by reference herein in its entirety.
Although prior art adaptive matrix decoders are intended to reduce crosstalk in the reproduced signals and more closely replicate the source signals, the prior art has done so in ways, many of which being complex and cumbersome, that fail to recognize desirable relationships among intermediate signals in the decoder that may be used to simplify the decoder and to improve the decoder's accuracy.
Accordingly, the present invention is directed to methods and apparatus that recognize and employ heretofore unappreciated relationships among intermediate signals in adaptive matrix decoders. Exploitation of these relationships allows undesired crosstalk components to be cancelled easily, particularly by using automatic self-cancelling arrangements using negative feedback.